Power and free conveyor systems for moving bulky items through a manufacturing or assembly plant are well known. Such power and free conveyors include a "power" and a "free" conveyor track, generally disposed vertically with respect to each other. Operating within the power track is an endless drive chain with pusher members periodically attached to the chain and extending toward the free track. These pusher members are oriented to engage a drive dog or actuator on a drive trolley operating within the free track. While the pusher members are generally fixed in position relative to the drive chain, the drive dogs on the drive trolleys are typically selectively retractable.
The free track generally follows the same path as the power track(s) but is spaced vertically relative thereto. As originally implemented, power and free conveyor systems were suspension systems with loads suspended from trolleys or carriers operating in the free track and with the power track disposed above the free track. These suspension systems have reached a high degree of sophistication and can include features such as the ability to stop and accumulate free trolleys in specific accumulating areas and transfer zones which include intersections where loads can be transferred between non-synchronous conveyor systems.
More recently, in response to the specific requirements of the automobile industry, floor mounted or "inverted" power and free systems have been developed. In these inverted systems, the power track and the free track are disposed beneath the floor of the factory, with the free track positioned above the power track. A plurality of load carriages are attached to the free trolleys through a slot in the factory floor. Each load carriage is usually attached to two or more free trolleys with the load carriage being disposed above the floor and driven along the conveyor path by associated free trolleys.
These inverted systems have the capability of handling bulkier and heavier loads, such as automobile chassis, while minimizing many dangerous conditions found in suspension systems. For example, inverted systems allow workers to safely climb on and off of the load carriages and they eliminate the danger inherent in the swinging loads of suspension systems.
At the same time, the development of inverted systems has presented a new and unique series of problems to designers. In a large factory, a single power and free conveyor can run for a mile or more. Within the length of the conveyor are a myriad of different assembly stations, many of which operate at different speeds, and thus require different drive chains. A longstanding problem in the design of power and free conveyors is the transfer of trolleys between non-synchronous separate drive chains operating at different speeds while avoiding mechanical interference and resultant jamming of the drive chains and trolleys.
One approach to an inverted power and free conveyor design for facilitating transfer of trolleys between actuator drive chains is described in U.S. Pat. No. 4,616,570 to Dehne ("the '570 patent"). In the '570 patent, each drive trolley includes a holdback dog and a drive dog which form a portion of a driving actuator which is vertically movable between operative and non-operative positions. The driving actuator is biased to the operative position. The drive dog is a so-called "wide dog" design including a pair of wings extending substantially on either side of a center portion of the trolley. The holdback dog is spaced from the drive dog and is considerably narrower than the drive dog. Separate non-jamming cam surfaces are formed on the front edge of the drive dog, extending along the entire width of the wings, and the rear edge of the holdback dog such that actuators striking the cam surfaces from either direction cause the driving actuator to be driven toward the non-operative position, thus preventing the jamming of the drive trolley. When the drive trolley is being driven normally by a single drive chain, a drive member within the chain engages the drive trolley between the drive dog and the holdback dog and pushes or pulls the drive trolley, along with an attached automobile body carriage and other trolleys, along the conveyor path. The drive members are generally T shaped, but are much narrower than the wide dog drive dogs on the drive trolley. When the drive trolley is to be transferred between synchronous or non-synchronous power tracks at a transfer zone, two power tracks, i.e. both a "delivering" and a "receiving" power track, are positioned in parallel, side-by-side beneath the free track. Within the transfer zone, the receiving power track is positioned slightly lower than the delivering power track. With this arrangement, when a drive trolley enters a transfer zone, the pusher members on the delivering power track engage one wing of the wide dog drive dog while the pushing members on the receiving power track engage the opposite wing. With the receiving power track being positioned lower than the delivering power track, should there be a mechanical conflict between actuators on the two chains, the receiving pusher member will engage a cam surface on the drive dog wing, thus harmlessly pushing the driving actuator on the drive trolley upward and allowing the receiving actuator to pass beneath the drive dog. Meanwhile, the pushing member on the delivering power track, with its slightly elevated position, will remain engaged with its respective drive dog wing, thus continuing to push the drive trolley through the transfer zone. As the drive trolley nears the end of the transfer zone, the power tracks diverge, with the receiving power track being shifted into position in alignment with the free track as the delivering power track diverges from the free track until only the receiving actuators engage the drive trolley drive dog.
The system taught by the '570 patent works to prevent mechanical conflict in transfer zones within which power tracks can be oriented in parallel. However, to function properly, transfer zones in the '570 patent must include two power tracks arranged in parallel position for a considerable length, thus requiring relatively long transfer zones. Such long transfer zones are not always practical in factory designs where it is often desirable for the length of a transfer zone to be minimized. In such short transfer zones, the delivering and receiving power tracks often approach each other at a considerable angle and only encounter each other tangentially. In such a transfer zone, there is not enough interaction between the two power tracks to allow reliable transfer of a wide dog drive trolley.
It is clear then, that a need exists for an improved design for a power and free conveyor transfer system which allows drive trolleys to be reliably and efficiently transferred between different non-synchronous power tracks at a transfer zone. Such a system should achieve reliable and jam-free transfers between delivering and receiving power tracks in short transfer zones wherein the power tracks approach each other at a relatively large angle and encounter each other only tangentially in the transfer zone.